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Section 3: Design Frequency

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Concept of Frequency

As with other natural phenomena, occurrence of flooding is governed by chance. The chance of flooding is described by a statistical analysis of flooding history in the subject watershed or in similar watersheds. Because it is not economically feasible to design a structure for the maximum possible runoff from a watershed, the designer must choose a design frequency appropriate for the structure.

The expected frequency for a given flood is the reciprocal of the probability or chance that the flood will be equaled or exceeded in a given year. For example, if a flood has a 20 percent chance of being equaled or exceeded each year, over a long period of time the flood will be equaled or exceeded on an average of once every five years. This is called the return period or recurrence interval (RI). Thus the exceedance probability equals 100/RI. The following table lists the probability of occurrence for the standard design frequencies.

Anchor: #i1010271Frequency versus Probability

Frequency (Years)

Probability (%)

2

50

5

20

10

10

25

4

50

2

100

1


The five-year flood is not one that will necessarily be equaled or exceeded every five years. There is a 20 percent chance that the flood will be equaled or exceeded in any year; therefore, the five-year flood could conceivably occur in several consecutive years. The same reasonin